1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical fixing members, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip.
One specific type of fuser employed in the fixing device is an endless belt looped for rotation around a stationary pipe of thermally conductive metal internally heated with a heater accommodated within its hollow interior. As the fuser belt rotates, the metal pipe conducts heat from inside to its outer circumference, which then transfers heat to the length of the rotating belt. Using the internally heated metal pipe allows for heating the fuser belt swiftly and uniformly, resulting in short periods of warm-up time and first print time required to complete an initial print job upon startup, and high immunity against printing failures caused by insufficiently heating of the fuser member in high speed application.
Various methods have been proposed to provide a stable, efficient structure for a fuser assembly employing a fuser belt looped around a heat pipe.
For example, one conventional fixing device employs a generally cylindrical, open-sided heat pipe formed by bending a sheet of thermally conductive material into a rolled configuration with a substantially C-shaped cross-section, around which a fuser belt is looped for rotation. According to this method, the heat pipe has a separate fuser pad held stationary in its side opening, outside the pipe hollow and inside the belt loop, with adequate spacing left between adjoining surfaces of the heat pipe and the fuser pad. In use, the heat pipe is disposed with the open side facing a rotatable pressure member, which is pressed against the fuser pad through the fuser belt to form a fixing nip.
Another conventional method also proposes a similar open-sided heat pipe with a fuser belt looped therearound and a separate fuser pad accommodated in the pipe opening, which is additionally equipped with a reinforcing member held stationary inside the heat pipe to strengthen and support the fuser pad subjected to pressure from a pressure member.
Using the open-sided heat pipe in combination with the fuser pad allows for high mechanical stability of the fuser assembly, since it enables the heat pipe to operate in isolation from pressure from the pressure member, which can thus maintain its generally cylindrical configuration without bending or bowing away from the fixing nip. Such protection against deformation is particularly important where the heat pipe is thin-walled to obtain high thermal efficiency in the fixing device. Mechanical stability of the open-sided fuser assembly is enhanced by adding reinforcement to the fuser pad, which ensures consistent contact between the fuser pad and the pressure member to maintain uniform distribution of pressure across the fixing nip
One problem encountered when using the open-sided heat pipe is that the heat pipe tends to lose its generally cylindrical shape, with the gap between the opening edges widened to deform the C-shaped cross-section. Such deformation is attributed to elastic recovery of the roll material after bending, known in the art as “springback”, and tends to occur at the longitudinal center rather than the longitudinal ends of the heat pipe which are typically fastened to a frame or support of the fixing device. The effects of elastic recovery are unacceptable where the heat pipe is formed of an extremely thin sheet of material for obtaining maximum thermal efficiency.
If not corrected, deformation of the heat roll can cause various defects due to interference or mis-coordination between the fuser belt and the heat roll, such as the belt getting damaged or making noise by excessively rubbing against the fuser roll, or running out of track by slipping off the roll surface.